1. Field of the Invention
This invention relates to a novel curled cord, and more particularly, it relates to a curled cord with excellent practicality for use, which is lightweight and compact, and has superior elasticity and restorability to its original configuration.
2. Brief Description of the Prior Art
A variety of conventional curled cords are known in which a conductor is covered with an insulating material, and then covered with a protective sheath material. For example, there are known curled cords with natural rubber or synthetic rubber used for the sheath material and curled cords with thermoplastic polymers used for the sheath material, such as vinyl chloride polymer, polyethylene, polyamide, polypropylene, and polyether.ester elastomers.
Also, the use of vinyl chloride polymer, polyethylene, polyamide, polypropylene, cross-linked polyvinyl chloride, cross-linked polyethylene, polyester.ether elastomers, etc., as insulating covering materials is known in the art.
The constitution and characteristics of curled cords suitable for use with telephones and the like have been disclosed in, for example, Japanese Laid-Open Patent Application No. 62-17909. The curled cords disclosed therein have a plurality of core wires comprising a conductor covered with an insulating material, the outside of which is further covered with a sheath layer made of, for example, a synthetic resin.
Also, a covered wire for the production of a curled cord with a small permanent stretch has been disclosed in Japanese Patent Publication No. 58-1484 that comprises as its basic material a polyester elastomer having a specific composition, that is, a specific copolyether ester elastomer.
In general, the basic functions of curled cords are:
(1) The capability to be stretched a moderate amount, PA1 (2) The capability to return satisfactorily to the form of a coiled spring, and PA1 (3) The property of not developing twists or reverse curls, so that the appearance is satisfactory. PA1 (4) Good insulating properties of the insulating covering layer, and PA1 (5) The capability of undergoing extrusion molding two times to form an insulating covering layer and a sheath layer.
Additional properties required are:
At present, as a sheath material for curled cords that is generally used, soft vinyl chloride is mainly used in view of its pliability, workability, and cost. In order to improve the restorability of curled cords to their original configuration, the spring constant, the pliability at low temperatures, and the like, many references including the patent publication No. 58-1484 mentioned above give various suggestions, but a substitute for vinyl chloride has not been provided yet.
A curled cord with vinyl chloride polymer used for the sheath material is widely used because of its excellent economy and flame-retardance. However, such a curled cord has poor heat-resistance and the holding of its curl disappears at high temperatures. Thus, in certain circumstances, such cords do not serve as curled cords. In addition, the surface of such cords is readily stained, and the curl may stretch out gradually by its own weight. Also, because the capability to retain the curl is poor, the length of the curled cord may be too long when the cord itself is not being stretched.
A curled cord in which the sheath material is polyester.ether elastomer has advantages including excellent retention of its curl and superior restorability to its original configuration after being stretched. However, when the curled cord is colored a variety of colors other than black, and particularly when the colors are pale, the weather-resistance is unsatisfactory, and when used out of doors, particularly at high temperatures in direct sunlight, there is the problem that loss of function as a protective covering may occur. Such a curled cord has superior elasticity for a restoration to its original configuration, and excellent retention of its curl, but when a cord in curled configuration is going to be stretched out and used, much force is needed to stretch it. For example, when the cord is used for a telephone or the like, there is the disadvantage that the telephone will be pulled toward the user. Also, compared to vinyl chloride polymer, polyester.ether elastomer is flammable, which is a disadvantage.
Conventional curled cords have the superior qualities described above in a variety of their properties. However, there are problems in that the various properties listed below are not retained in good balance: that the retention of the curl needed for curled cords for practical use be good; that the restoration to its original configuration be excellent; that the curled cord can be stretched to an appropriate length by the use of a small amount of force; and that the cord has flame-retardant properties, excellent weather-resistance, and the like. Also, in order to provide a curled cord for practical use, it must be economical and have superior productivity.